The present invention relates to controlling the supply of bodyfeed to a filter for removing solids from a liquid stream and relates particularly but not exclusively to removing yeast and haze from a beer stream.
Breweries around the world have been primarily using kieselguhr, or diatomaceous earth (DE), filters for primary filtration of xe2x80x9cgreenxe2x80x9d beer for over 50 years. DE is essentially comprised of skeletal remains of microscopic organisms which are mined and then processed to produce highly porous diatoms. DE is ideal for removing yeast and haze solids from xe2x80x9cgreenxe2x80x9d beer. Alternatively, DE may be substituted with perlites, silica gels or alterative filter aids.
The basic primary filtration units used are horizontal leaf, vertical leaf or candle filters. Irrespective of the filter type used, the mode of filtration of xe2x80x9cgreenxe2x80x9d beer uses the same basic principles. The filter septum or screen used in each type of filter typically has apertures of approximately 50 xcexcm. The DE acts as a filter aid for removing solids from the beer. Precoating the filter with a course-grade DE, and then using a fine-grade DE as a bodyfeed, can remove beer suspended solids down to a size of approximately 1 xcexcm.
The correct application of bodyfeed maintains a microstructure with high porosity and high effective bed voidage (EBV). EBV describes the capacity of filter cake to trap beer solids, which influences the clarifying capability of the filter aid.
Two of the major challenges with beer filtration are, determining the correct grade of filter aid to use, and applying bodyfeed at the correct rate. This depends largely on the characteristics of the green beer to be processed.
Bodyfeed should be applied at an appropriate rate to maintain adequate porosity, maintain beer clarity and minimise filter aid usage, thereby maximising filter run times and minimising costs.
Savings may be achieved by reduced filter aid consumption, reduced filter aid disposal costs (which are destined to rise substantially in the future) and less frequent filter down times. There are also secondary benefits in the form of increased productivity and quality control. For example, in existing systems a number of filters are run in parallel and as they become clogged with material they must be taken off-line and cleaned. Thus, if they can be kept in service for longer by efficient application of bodyfeed, there is less down time.
In known filtration processes bodyfeed is generally controlled manually, if at all. In many situations, the dosing pumps are set to high bodyfeed rates and are not changed during filtration runs. Alternatively, a human operator controls the dosing pump guided by their understanding of the process. An operator is required to make a judgement as to whether the dosing should be varied based on their own experience.
From the applicant""s experience, it is known to be desirable to maintain a constant increase of pressure drop across the filter of 20-40 kPa/hr. Pressure drop increases of less than 20 kPa/hr indicate overdosing (or too course a filter aid is being used and the structure is too porous) and the dosing action needs to be reduced. Pressure drop increases of greater than 40 kPa/hr indicate that the EBV is too low (or too fine a filter aid is being used and the structure is not sufficiently porous) and the dosing action needs to be increased.
One known system for controlling bodyfeed dosing relies on measuring the turbidity of the beer stream as it approaches the filter and on modelling the EBV of the DE filter aid. This system is very cumbersome and requires substantial operator input to the mathematical model of the EBV.
Problems arise when attempting to control the filter dosage because significant variations can occur in the characteristics of the DE. Furthermore, turbidity probes for measuring upstream turbidity of the beer are not able to discriminate between different beer solids, which have very different effects on filtration. A control system based on such measurements is generally unsatisfactory.
Therefore, it would be desirable to provide a control system which provides more accurate control of the dosing of the DE bodyfeed to the filter and which does not require experienced operator control.
Accordingly, there is provided a control system for controlling the supply of bodyfeed to a filter for removing solids from a liquid stream, including:
turbidity determining means for determining turbidity of the liquid stream upstream of the filter;
pressure drop change determining means for determining the rate of pressure drop change across the filter; and
a controller having dose change calculation means for calculating a dose change from a determined turbidity and a determined rate of pressure drop change, and altering means for altering the supply of bodyfeed to the filter in accordance with a calculated dose change.
Preferably, the control system further includes trend determining means for determining a trend of the rate of pressure drop change and wherein the dose change means calculates the dose change from the determined trend of rate of pressure drop change in addition to the determined turbidity and determined rate of pressure drop change.
Preferably, the dose calculation means is a fuzzy controller.
Preferably, the fuzzy controller calculates the dose change in accordance with the following set of rules:
IF rate of pressure drop change is low AND trend of rate of pressure drop change is increasing AND turbidity is low THEN dose change is zero
IF rate of pressure drop change is low AND trend of rate of pressure drop change is steady AND turbidity is low THEN dose change is xe2x88x92small
IF rate of pressure drop change is low AND trend of rate of pressure drop change is decreasing AND turbidity is low THEN dose change is xe2x88x92large
IF rate of pressure drop change is ideal AND trend of rate of pressure drop change is increasing AND turbidity is low THEN dose change is +small
IF rate of pressure drop change is ideal AND trend of rate of pressure drop change is steady AND turbidity is low THEN dose change is zero
IF rate of pressure drop change is ideal AND trend of rate of pressure drop change is decreasing AND turbidity is low THEN dose change is xe2x88x92small
IF rate of pressure drop change is high AND trend of rate of pressure drop change is increasing AND turbidity is low THEN dose change is +large
IF rate of pressure drop change is high AND trend of rate of pressure drop change is steady AND turbidity is low THEN dose change is +small
IF rate of pressure drop change is high AND trend of rate of pressure drop change is decreasing AND turbidity is low THEN dose change is zero
IF turbidity is high THEN dose change is +very large
Preferably, the dose change calculation means includes aggregating means for aggregating the rules when more than one rule is fired so that said dose change is based on the aggregation of said rules.
Preferably, said aggregating means aggregates said rules in accordance with weightings allocated to each of said rules.
Preferably, the rule where turbidity is high is given a greater weighting than the rules where turbidity is low.
Preferably, the rule where turbidity is high is given a weighting approximately ten times the weighting of the rules where turbidity is low.
Preferably, the liquid stream is a beer stream.
The invention also provides a method for controlling supply of bodyfeed to a filter for removing solids from a liquid stream, including:
determining turbidity of the liquid stream upstream of the filter;
determining the rate of pressure drop change across the filter; and
calculating a dose change from the determined turbidity and the determined rate of pressure drop change and altering the supply of body feed to the filter in accordance with the calculated dose change.
Preferably, the method further includes determining a trend of the rate of pressure drop change and the dose change is calculated from the determined trend of rate of pressure drop change in addition to the determined turbidity and determined rate of pressure drop change.
Preferably, the dose change is calculated by a fuzzy controller.
Preferably, the fuzzy controller calculates the dose change in accordance with the following set of rules:
IF rate of pressure drop change is low AND trend of rate of pressure drop change is increasing AND turbidity is low THEN dose change is zero
IF rate of pressure drop change is low AND trend of rate of pressure drop change is steady AND turbidity is low THEN dose change is xe2x88x92small
IF rate of pressure drop change is low AND trend of rate of pressure drop change is decreasing AND turbidity is low THEN dose change is xe2x88x92large
IF rate of pressure drop change is ideal AND trend of rate of pressure drop change is increasing AND turbidity is low THEN dose change is +small
IF rate of pressure drop change is ideal AND trend of rate of pressure drop change is steady AND turbidity is low THEN dose change is zero
IF rate of pressure drop change is ideal AND trend of rate of pressure drop change is decreasing AND turbidity is low THEN dose change is xe2x88x92small
IF rate of pressure drop change is high AND trend of rate of pressure drop change is increasing AND turbidity is low THEN dose change is +large
IF rate of pressure drop change is high AND trend of rate of pressure drop change is steady AND turbidity is low THEN dose change is +small
IF rate of pressure drop change is high AND trend of rate of pressure drop change is decreasing AND turbidity is low THEN dose change is zero
IF turbidity is high THEN dose change is +very large
Preferably, dose change is calculated by aggregating the rules when more than one rule is fired so that said dose change is based on the aggregation of said rules.
Preferably, said rules are aggregated in accordance with weightings allocated to each of said rules.
Preferably, the rule where turbidity is high is given a greater weighting than the rules where turbidity is low.
Preferably, the rule where turbidity is high is given a weighting approximately ten times the weighting of the rules where turbidity is low.
Preferably, the liquid stream is a beer stream.